The present invention relates to titanium alloys usable at high temperatures, particularly those of the TiAl gamma phase type. Titanium alloys have found wide use in gas turbines in recent years because of their combination of high strength and low density, but generally, their use has been limited to below 600.degree. C. by inadequate strength and oxidation properties. At higher temperatures, relatively dense iron, nickel and cobalt base superalloys have been used. However, lightweight alloys are still most desirable, as they inherently reduce stresses when used in rotating components.
While major work was performed in the 1950's and 1960's on lightweight titanium alloys for higher temperature use, none have proved suitable for engineering application. To be useful at higher temperatures, titanium alloys need the proper combination of properties. In this combination are properties such as high ductility, tensile strength, fracture toughness, elastic modulus, resistance to creep, fatigue, oxidation, and low density. Unless the material has the proper combination, it will fail, and thereby be use-limited. Furthermore, the alloys must be metallurgically stable in use and be amenable to fabrication, as by casting and forging. Basically, useful high temperature titanium alloys must at least outperform those metals they are to replace in some respects, and equal them in all other respects. This criterion imposes many restraints and alloy improvements of the prior art once thought to be useful are, on closer examination, found not to be so. Typical nickel base alloys which might be replaced by a titanium alloy are INCO 718 or INCO 713. The density-corrected stress rupture capabilities of these materials are shown in FIG. 1 together with the best commercially available titanium base alloys. It is seen that prior titanium alloys had inferior properties to nickel alloys. Alloys of the present invention, to be discussed below, are also known on the Figure.
Heretofore, a favored combination of elements for higher temperature strength has been titanium with aluminum, in particular alloys derived from the intermetallic compounds or ordered alloys Ti.sub.3 Al (.alpha..sub.2) and TiAl (.gamma.). It should be evident that the TiAl gamma alloy system has the potential for being lighter, inasmuch as it contains more aluminum. Laboratory work in the 1950's indicated these titanium aluminide alloys had the potential for high temperature use to about 1000.degree. C. But subsequent engineeing experience with such alloys was that, while they had the requisite high temperature strength, they had little or no ductility at room and moderate temperatures, i.e., from 20.degree. to 550.degree. C. Materials which are too brittle cannot be readily fabricated, nor they can they withstand infrequent but inevitable minor service damage without cracking and subsequent failure. They are not useful engineering materials to replace other base alloys.
There are two basic ordered titanium aluminum compounds of interest-Ti.sub.3 Al and TiAl which could serve as a base for new high temperature alloys. Those well skilled recognize that there is a substantial difference between the two ordered phases. Alloying and transformational behavior of Ti.sub.3 Al resemble those of titanium as the hexagonal crystal structures are very similar. However, the compound TiAl has a tetragonal arrangement of atoms and thus rather different alloying characteristics. Such a distinction is often not recognized in the earlier literature. Therefore, the discussion hereafter is largely restricted to that pertinent to the invention, which is within the TiAl gamma phase realm, i.e. TiAl, 50Ti-50Al atomically, and about 65Ti-35Al by weight.
With respect to the early titanium alloy work during the 1950's, several U.S. and foreign patents were issued. Among them were Jaffee U.S. Pat. No. 2,880,087, which disclosed alloys with 8-34 weight percent aluminum with additions of 0.5 to 5% beta stabilizing elements (Mo, V, Cb, Ta, Mn, Cr, Fe, W, Co, Ni, Cu, Si and Be). The effects of the various elements were distinguished to some extent. For example, vanadium from 0.5-50% was said to be useful for imparting room temperature tensile ductility, up to 2% elongation, in an alloy having 8-10% aluminum. But with the higher aluminum content alloys--those closest to the gamma TiAl alloy--ductility was essentially non-existent for any addition. Likewise, Gullett in U.S. Pat. No. 2,881,105 mentions a 6-20 weight percent aluminum alloy strengthened by adding up to 2% vanadium.
Jaffee in Canada Pat. No. 596,202 mentions other useful alloys of less than 8 weight percent aluminum while indicating the problem of hot workability for higher aluminum contents. The problem is said to be overcome by the addition of the aforementioned beta stabilizing elements in combination with germanium (an alpha stabilizer). Jaffee discloses the utility of carbon in 0.05 to 0.3%, to improve the hot strength of high (up to 32%) aluminum containing alloys of his particular invention. Similar art is revealed in Finlay et al. Canada Pat. No. 595,980, wherein it is also said that other elements, such as molybdenum, manganese, vanadium, columbium, and tantalum are useful. But a review of the data in the 595,980 patent and specification indicates little basis for distinguishing between the elements and shows a prevalence of "zero" tensile elongations at room temperature. Jaffee in Canada No. 621,884 discloses aluminum contents of 34 to 46 weight percent. Noted are the alloys' lack of responsiveness to heat treatment. No data on tensile elongation is given, but is inferred that 34- 46% aluminum gives maximum ductility based on the low hardness values. (This is obviously an incorrect inference as our work shows Ti-38%Al has a low hardness and no tensile ductility at ambient temperature). Both alpha and beta promotors are indicated as desirable additions in 0.1 to 5% amounts but no suggestion is made for selection within its broad group. In early published work, such as "Ti-36 Pct Al as a Base for High Temperature Alloys", by McAndrew and Kessler in Transactions AIME, Vol. 206, p. 1384ff (1956), many TiAl compositions with additions including niobium and tantalum were investigated, showing improved creep and limited improvement in room temperature properties. Other investigators reported on hardness and lattice parameters for alloys containing zirconium and yttrium. More fundamental studies under U.S. Air Force sponsorship were carried out to investigate alloying fundamentals in the mid-1970's. Air Force and private work indicated that Zr, Ni, In, and Ga increased TiAl strength but not ductility. During the past twenty years, there also has been work on various ternary systems including Ti-Al-V. For example, see Kornilov et al in "Metal Science and the Application of Titanium and its Alloys" Volume 8, 92 Nauka Press, Moscow (1965). Most of this work has been concerned with phase identification and stability ranges rather than the development of useful alloys.
Despite these past revelations, TiAl alloys having engineering and commercial utility have not been identified and have not made available. This can be attributed to the limited evaluations and necessarily broad approaches of the past. The prior art teaches some broad but contradicting approaches. There is better understanding today and considerable ongoing research, of which this invention is a product. But it is not yet responsible to declare a comprehensive insight into obtaining high temperature strength and low temperature ductility in intermetallic titanium alloys. As will be shown below, the broad teachings of the past are now found not to be entirely accurate and useful. For example, all transition elements were considered similar in effect in much of the prior art.